The user can control the value of output frequency by adjusting the parameters in the control circuit. The diodes in the above diagram are used to represent the direction of flow of current. The positive switching circuit always sources current into the load and the negative switching circuit always sinks current from the load. One set is placed straight while the other is placed in anti-parallel direction as shown on the picture below.
This control circuit will be responsible for triggering the SCRs. Let us analyse the wave form below to understand how frequency is stepped down using a CCV. The waveform of the supply voltage frequency is denoted by Vs and the wave form of the output voltage frequency is denoted by Vo.
So to do that for the first two cycles of the supply voltage we will use the positive Bridge rectifier and for the following next two cycles we will use the negative bridge rectifier. Thus we have four positive pulses in the positive region and then four in the negative region as shown in the output frequency waveform Vo. The current waveform for this circuit will be the same as voltage waveform since the load is assumed to be purely resistive. Although the magnitude of the waveform will change based on the value of resistance of the load.
This output frequency can be controlled by varying the triggering mechanism in the control circuit. The circuit also looks very similar except we will need 6 SCR in each set of Rectifier since we have to rectify the 3 Phase AC voltage. Again the gate terminals of the SCR will be connected to the control circuit for triggering them and the same assumptions are made again to understand the working easily.
This type of cycloconverter circuit employs two single-phase fully-controlled converters known as positive and negative converters. These two converters are connected in antiparallel. The positive converter supplies load current when the output undergoes the first positive half cycle. Whereas, in the negative half of the output cycle negative converter supplies the load current.
It should ensure that two converter circuits should not be in conduction at the same time, otherwise a short circuit will take place at the input terminals. When there is a need for output voltage waveform vary nearly to sinusoidal wave, three-phase to single-phase cycloconverters are best suited. Since single-phase to single-phase cycloconverters can supply non-sinusoidal output voltage, a pure sinusoidal output voltage can be fabricated from a three-phase supply using three-phase to single-phase cycloconverters.
The below shows the basic circuit configuration of a three-phase to single-phase cycloconverter. The circuit incorporates two three-phase half-wave converters with antiparallel connections.
One half-wave converter circuit forms the positive group that conducts for positive load currents, while the other half-wave converter circuit forms the negative group that conducts for negative load currents. The below waveform shows the single-phase output voltage positive half-cycle fabricated from the three-phase input voltage. The basic function remains the same i.
A single phase to single phase cycloconverter may either be mid-point type or bridge type step-up or step-down cycloconverter. In this article, we will understand the circuit configuration and working of single phase to single phase bridge type step-up cycloconverter. The circuit diagram of a single phase to single phase bridge type step-up cycloconverter is shown below. It consists of a total of eight number of thyristors , P1 to P4 i. The output voltage and frequency of a cyclo-converter can be varied continuously and independently using a control circuit.
Therefore, unlike other converters, it is a single stage frequency converter. Cyclo-converters are constructed using naturally commutated thyristors with inherent capability of bidirectional power flow. These can be single phase to single phase, single phase to three- phase and three-phase to three phase converters. So the control circuit implementation is not simple because large number of SCRs, typically 4 or 8 SCRs for single phase and 36 for three- phase supply.
In case of step-down cyclo-converter, the output frequency is limited to a fraction of input frequency, typically it is below 20Hz in case 50Hz supply frequency. In this case, no separate commutation circuits are needed as SCRs are line commutated devices. Such circuits are relatively very complex. Therefore, majority of cyclo-converters are of step-down type that lowers the frequency than input frequency.
Besides the frequency control, cyclo-converter output voltage can be varied by applying phase control technique. These can be used to provide either fixed frequency output from variable frequency input value or variable frequency output from fixed frequency input.
These are mainly used in very high power, low speed AC motors and traction systems, especially low frequency three-phase to single phase systems. The equivalent circuit of a cyclo-converter is shown in figure below.
Here each two quadrant phase controlled converter is represented by a voltage source of desired frequency and consider that the output power is generated by the alternating current and voltage at desired frequency. The diodes connected in series with each voltage source represent the unidirectional conduction of each two quadrant converter. If the output voltage ripples of each converter are neglected, then it becomes ideal and represents the desired output voltage.
DIY Project Kit : Cyclo Converter using Thyristors » If the firing angles of individual converters are modulated continuously, each converter produces same sinusoidal voltages at its output terminals. So the voltages produced by these two converters have same phase, voltage and frequency. The average power produced by the cyclo-converter can flow either to or from the output terminals as the load current can flow freely to and from the load through the positive and negative converters.
Therefore, it is possible to operate the loads of any phase angle or power factor , inductive or capacitive through the cyclo-converter circuit. Due to the unidirectional property of load current for each converter, it is obvious that positive converter carries positive half-cycle of load current with negative converter remaining in idle during this period.
Similarly, negative converter carries negative half cycle of the load current with positive converter remaining in idle during this period, regardless of the phase of current with respect to voltage.
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